The present invention relates to an improved magnetic path-forming member, and more particularly relates to a magnetic path-forming member especially suitable for use as a stator yoke in a brushless DC motor.
In a conventional brushless motor, driving armatures are mounted on a stator yoke so as to be interposed in the magnetic path of a rotor magnet and to urge the latter to rotate. A conventional stator yoke is typically made of magnetically permeable material, such as iron plate or electrical steel, so as to provide a path for magnetic flux emanating from the rotor magnet. The armature windings in such a conventional motor are wound on non-conductive (plastic) bobbins with flange portions adapted to abut the stator yoke in order to insulate the armature windings from the metal of the stator yoke.
While a thickness of only 0.1 mm would be sufficient for insulating purposes, the bobbin flange usually has a thickness of 0.5 mm, and cannot be manufactured with a thickness of less than 0.3 mm due to difficulties in molding of the bobbin.
Leakage of magnetic flux occurs in all such motors in the gap between the rotor magnet and the stator yoke, and the amount of flux loss is substantially proportional to the cube of the length of gap spacing. The flux leakage due to interpositioning of the bobbin flange in the path of the magnetic flux significantly reduces the power capacity of the motor. Thus, it is evident that the power of a DC brushless motor can be increased by eliminating the necessity for the bobbin flanges.